The present invention relates generally to acousto-optic devices, such as acousto-optic modulators (AOMs) and acousto-optic deflectors, and in particular to an acousto-optic device that utilizes a pulsed light source to generate acoustic waves.
Acousto-optic interaction occurs in all optical media when an acoustic wave and a laser beam are present in the medium. When an acoustic wave is launched into the optical medium, it generates a refractive index wave that behaves like a sinusoidal grating. An incident laser beam passing through this grating will be diffracted into several orders. With appropriate design, the first order beam has the highest efficiency. Its angular position is linearly proportional to the acoustic frequency, so that the higher the frequency, the larger the diffracted angle.
Conventional devices exist that modulate and/or deflect a beam of light using acoustic waves. These devices are known as acousto-optic modulators (AOM""s) and/or acousto-optic deflectors (AOD""s). AOM""s and AOD""s contain a glass or glass-like material that is transparent to the incident light beam. The beam of light enters one face of the glass material and exits a second face. A third face, normal to the propagation of the beam of light, has a piezoelectric material, such as lithium niobate, attached. Electrodes are deposited on the lithium niobate. High frequency sinusoidal AC electric drive signals are sent to the electrodes. The electrode in turn causes expansion and/or contraction of the piezo-electric material. The expansion/contraction of the piezo-electric material causes a sinusoidal force to be applied to the transparent (glass) material. The sinusoidal force becomes a wave traveling through the glass and is commonly referred to as a sound wave or acoustic wave. The frequency of the acoustic wave is related to the frequency of the sinusoidal AC electric drive signal. Sound frequencies from 1 kilohertz to 1 gigahertz are possible.
In the case of the AOM, the sound wave travels through the transparent material and the sound wave frequency is constant. The sound wave causes variations in density within the transparent material and causes the light beam to diffract. The diffracted beam of light leaves the transparent material at a different angle than the un-diffracted light beam. By turning the AC electric drive signal on or off, the diffracted beam can be modulated.
In the case of the AOD, the frequency of the AC electric drive signal is modulated. Varying the frequency of the AC electric drive signal causes the sound wave frequency in the glass to change. By varying the sound frequency, the diffracted light beam angle also varies. By applying a varying frequency AC electric drive signal the output light beam is made to scan from one angular output to another. The presence or absence of the AC electric drive signal is used to switch the light beam xe2x80x9conxe2x80x9d or xe2x80x9coff.xe2x80x9d
The performance of conventional acousto-optic modulators is limited by the use of the piezo-electric material and electrode. The modulation performance, deflection performance, and efficiency of an AOM or AOD is in part determined by the shape of the electrode on the piezo-electric material. The shape of the electrode on the piezo-electric material determines the shape of the sound field propagating through the glass material. The shape of the sound field in the glass effects the efficiency and alignment sensitivity of the AOM and AOD. Thus, the electrodes must be precisely shaped. This may be difficult due to the complicated geometry of the electrode. If it is desired to change the shape of the sound field, an entirely new electrode must be prepared.
Additionally, the characteristics of the drive electronics also effect the modulation performance, deflection performance, and efficiency of the AOM or AOD. The voltage, impedance and drive power capabilities of the particular power supplies used to drive the electrodes must be carefully matched to the impedance and other electrical characteristics of the piezo-electric material. As a result, the drive electronics often include complex circuits. It is also difficult to generate very high frequency acoustic waves with conventional electrodes as a result of the electrical capacitance of the piezo-electric material. Lastly, the piezo-electric material must be attached to the glass material. This step may be difficult to perform since it involves pressure under vacuum and requires low melt point metals to cold weld the piezo-electric material to the glass material.
Accordingly, what is desired is an acousto-optic device that has good performance, that provides greater flexibility to produce sound fields of different shape, and is capable of achieving high acoustic wave frequencies but that does not utilize an electrode and piezo-electric material to generate acoustic waves.
The present invention relates to an acousto-optic device that does not use piezo-electric materials and therefore does away with the sound field shape constraint caused by the shape of the electrode.
In a first aspect of the invention, an acousto-optic device comprises a first light source for producing a light beam, a transparent material capable of transmitting the light beam along an optical path through the transparent material, a light-absorbing material applied to the transparent material, and a pulsed light source capable of directing a pulsed light beam at the light-absorbing material so as to produce acoustic waves within the transparent material that cross the optical path.
In another aspect of the invention, a method is provided for diffracting a light beam, comprising the steps of providing a transparent material, applying a light-absorbing material to the transparent material, transmitting a light beam along an optical path through the transparent material, and directing a pulsed light beam at the light-absorbing material to produce acoustic waves within the transparent material that cross the optical path, so as to diffract the light beam.
The present invention provides a significant advantage over the prior art by eliminating the conventional electrode and piezo-electric material used in conventional AOMs and AODs to generate the acoustic wave within the transparent material. Instead, by utilizing a pulsed laser beam, the present invention allows greater flexibility in design, since the shape of the sound front within the transparent material may be varied by simply changing the shape of the pulsed light beam applied to the light-absorbing material. Thus, the present invention eliminates the need to precisely shape the electrode. The invention also eliminates the need to match the impedance of the electrode and piezo-electric materials with the drive electronics.
The present invention finds utility in a variety of different applications. In one embodiment, the acousto-optic device may be used in a multi-channel device. In another embodiment, the acousto-optic device is used in an acoustic traveling wave lens. In yet another embodiment, the acousto-optic device is used to separate a primary laser beam from secondary satellite beams.
Yet another embodiment of the invention provides a method for selectively transmitting a light beam. A light-transmitting material is provided. A light-absorbing material is contacted to the light-transmitting material. A light beam is transmitted along an optical path through the light-transmitting material. A pulsed light beam is directed at the light-absorbing material to produce acoustic waves within the light-transmitting material that are co-axial with the optical path. This method results in selective reflection or transmission of the light beam through the light-transmitting material. This method allows the light beam to be filtered to a desired range of wavelengths, to be amplitude modulated, or to be spatially modulated.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.